This application generally relates to nuclear reactor fuel assemblies and more particularly relates to a nuclear fission reactor fuel assembly adapted to permit expansion of the nuclear fuel contained therein.
It is known that, in an operating nuclear fission reactor, neutrons of a known energy are absorbed by nuclides having a high atomic mass. The resulting compound nucleus separates into fission products that include two lower atomic mass fission fragments and also decay products. Nuclides known to undergo such fission by neutrons of all energies include uranium-233, uranium-235 and plutonium-239, which are fissile nuclides. For example, thermal neutrons having a kinetic energy of 0.0253 ev (electron volts) can be used to fission U-235 nuclei. Thorium-232 and uranium-238, which are fertile nuclides, undergo induced fission, with fast neutrons, which have a kinetic energy of at least 1 MeV (million electron volts). The total kinetic energy released from each fission event is about 200 MeV. This kinetic energy is eventually transformed into heat.
Moreover, the fission process, which starts with an initial source of neutrons, liberates additional neutrons as well as transforms kinetic energy into heat. This results in a self-sustaining fission chain reaction that is accompanied by continued energy release. That is, for every neutron that is absorbed, more than one neutron is liberated until the fissile nuclei are depleted. This phenomenon is used in a commercial nuclear reactor to produce continuous heat that, in turn, is beneficially used to generate electricity.
Fuel assembly expansion due to the aforementioned heat generation and also due to fission product release can occur in such processes. In this regard, fuel assemblies may undergo differential expansion, fuel rod creep that can increase incidence of fuel rod cladding rupture, fission gas pressure build-up, and swelling during reactor operation. This may increase the incidence of fuel pellet cracking and/or fuel rod bowing. Fuel pellet cracking may lead to fission gas release and cause higher than normal radiation levels. Fuel rod bowing may in turn lead to obstruction of coolant flow channels. Safety margins incorporated into the reactor design and precise quality control during manufacture can reduce these incidences or the system design can adapt systems to operate with such incidences.
In one approach to deading with fuel assembly expansion due to heat generation and fission gas release, U.S. Pat. No. 3,028,330 issued Apr. 3, 1962 in the name of Clarence I. Justheim, et al. and titled “Nuclear Fuel Elements Having An Autogenous Matrix And Method Of Making The Same” discloses a cellular carbonaceous matrix. The cells of the cellular matrix can contain fragments of a fissile material, which may be fertile isotopes of uranium enriched with fissionable isotopes. According to this patent, the cells are ordinarily of such size relative to the fission fragments as to allow for increase in volume of the latter resulting from thermal cycling and radiation damage. Although this patent discloses a cellular matrix having cells that allow for increase in volume of fission fragments, this patent does not appear to disclose a nuclear fission reactor fuel assembly, which is adapted to permit expansion of the nuclear fuel contained in the fuel assembly.
Another approach, disclosed in U.S. Pat. No. 3,184,392 issued May 18, 1965 in the name of Leslie Reginald Blake, et al. and titled “Fast Nuclear Reactor Fuel Elements” describes a nuclear reactor fuel element that comprises a body of porous, closed-cell, fissile nuclear fuel which by virtue of the porosity is of dispersed structure and which is enclosed within a cylindrical protective sheath. The fuel provides interstitial voids and the fuel element is only partially filled to leave a void above the fuel. According to this patent, the protective sheath is capable of withstanding an internal pressure of at least 10,000 psi at a temperature of 600° C. and the void above the fuel serves as expansion space for the fuel and also space for accommodation of fission products. Although this patent discloses a nuclear reactor fuel element that comprises a body of porous, closed-cell, fissile nuclear fuel, this patent does not appear to disclose a nuclear fission reactor fuel assembly, which is adapted to permit expansion of the nuclear fuel contained in the fuel assembly, as disclosed and claimed herein.